Clean elfvsb files left over from previous runs

Clean elfvsb files left over from previous runs

My mips-linux and mips64-linux testsuite runs have been failing a
bunch of visibility tests, seemingly randomly.  It turns out the
problem occurs when object files are left over in ld/tmpdir from a
previous run.

	* testsuite/ld-elfvsb/elfvsb.exp (visibility_run): Delete
	sh1p.o, sh2p.o, sh1np.o and sh2np.o before compiling.  Use
	remote_file host exists rather than file exists.